This invention relates to a process for removing fine suspensions of water-soluble solids from organic liquids which are more dense than water and which readily form an emulsion with water.
Various approaches have been used in the prior art to remove finely divided, suspended particulate material from organic liquids. The suitability of a particular approach depends both on the nature of the particulate material to be removed and the nature of the organic liquid. Certain processes have been shown to be more effective than others. Simple extraction and countercurrent extraction have long been known in the art. However, these methods possess serious disadvantages in that when it is desired to remove an extractable material from an organic liquid which readily forms an emulsion with water, water cannot be used as the extracting agent in an agitated system without the addition of some other material which will prevent the formation of an emulsion or cause a formed emulsion to resolve. The addition of an agent to aid in resolving such an emulsion has the disadvantage of adding additional material to the system which must eventually be removed and is inefficient because additional steps in the extraction process are required. Another class of separation processes, filtration, are also well known in the art. In general, alternate filtration approaches have been employed such as basket centrifugation and solid bowl centrifugation. Basket and solid bowl centrifugation are not suitable when high purity and efficiency is required because they can exhibit only 96% efficiency and 98.5 efficiency, respectively. Filtration per se, although effective in removing large particulate matter, is not entirely satisfactory when a liquid organic product with extremely fine particulate material suspended therein is involved because such particulate material cannot readily be removed by this expedient. Therefore, wash or extraction processes are often employed to purify such organic liquids.
Conventionally, in countercurrent extraction processes the more viscous phase is dispersed in the less viscous phase. This achieves small droplet size distribution, which normally correlates to better and more complete mass-transfer. This prior art approach, however, does not yield satisfactory results with heavy organic hydrocarbons containing extremely fine particulate matter suspended therein because an insufficient quantity of the particulate material is extracted.
U.S. Pat. No. 679,575 discloses a countercurrent method and apparatus for extracting matter from liquids. The apparatus includes an extraction column which has an inlet and an outlet at both the upper and lower portions of the column. The system is operated such that both the extracting liquid and the liquid containing matter to be extracted is introduced into the column in a finely divided form. This finely divided form is maintained throughout a substantial length of the column. Agitation means is provided for maintaining the two phases in intimate, finely divided contact. Such a system is unsuitable for extracting water soluble suspended particulate matter from a liquid which readily forms an emulsion with water.
U.S. Pat. Nos. 1,951,787; 2,273,915; 2,852,341; 4,221,658 and 4,424,131 disclose various modifications of this general type of process. However, none is practical for use with a liquid which readily forms an emulsion of water.
Countercurrent extraction of water-soluble particles from heavier-than-water organic liquids is normally carried out in a vertical extraction column with the conditions of the extraction controlled such that the liquid/liquid interface is maintained near the bottom portion of the column and thus the organic liquid is maintained as the discrete phase. The object of such prior art processes is to achieve small droplet size distribution which normally results in better, more complete mass transfer. However, when the prior art processes are carried out on highly viscous organic liquids which readily form an emulsion with water, removal of the water-soluble particles contained in the organic phase is not satisfactory.
It has been found, when the water-soluble particles are, e.g., solid NaCl, that in dissolving the particles from the organic phase using water, the resultant system is a three component system, viz., (1) a highly viscous organic liquid (2) a particulate phase suspended in the organic phase and (3) an immiscible aqueous phase. The prior art approach of maintaining the organic liquid as a discrete phase is unsatisfactory at least in part because the particulate phase is not free to move in the organic phase. Thus, adequate contact between the particles to be dissolved and the water is not achieved. In the case of an organic liquid which readily emulsifies with water, improved water-particle contact cannot be obtained by agitation since an emulsion results.
The present invention overcomes the prior art difficulties by maintaining the water as the disperse phase so that particle-water contact is carried out more efficiently.